1. Field of the Invention
The present invention relates to a balanced-type dielectric filter and a high frequency circuit using the filter, and more particularly, it relates to a balanced-type dielectric filter which can be used in combination with an integrated circuit (IC) or multiple ICs to form a high frequency circuit at a low loss without using a balanced-to-unbalanced converter or an unbalanced-to-balanced converter.
2. Description of the Related Art
Conventional dielectric filters are used in high frequency circuits. These dielectric filters are "unbalanced", and therefore are coupled to ICs within the high frequency circuits using balanced-to-unbalanced converters and unbalanced-to-balanced converters.
FIGS. 5(a), 5(b) and 6 show an example of a conventional dielectric filter 40, where FIG. 5(a) is a perspective view showing internal conductors using hidden lines, FIG. 5(b) is a second perspective view with the filter turned upside-down, and FIG. 6 is a cross-sectional view taken along the plane A--A' of FIG. 5(a).
As shown in FIG. 5(a), FIG. 5(b) and FIG. 6, the conventional dielectric filter 40 includes a dielectric block 31 formed around inner conductors 32-1 and 32-2. Each of the inner conductors 32-1 and 32-2 has a length approximately equal to 1/4 of a selected wavelength. The inner conductors 32-1 and 32-2 include wide portions 32-1a and 32-2a, respectively, and also include narrow portions. The filter 40 also includes outer conductors 33-1 to 33-4 formed on top, bottom and side surfaces of the dielectric block 31, and a short-circuiting conductor 34 formed on one end of the dielectric block 31. The filter 40 also includes an input terminal 35-1 and an output terminal 35-2 which are located on the side surfaces of the dielectric block 31 and include terminal electrodes 37-1 and 37-2, respectively. The input terminal 35-1 and output terminal 35-2 are respectively connected to the inner conductors 32-1 and 32-2 by connecting conductors 36-1 and 36-2. Finally, the input terminal 35-1 and output terminal 35-2 are electrically isolated from the outer conductors by insulating portions 38-1 and 38-2, respectively.
The dielectric block 31 is shaped approximately like a rectangular parallelepiped. The inner conductors 32-1 and 32-2, which have approximately square cross-sections, are disposed in parallel within the dielectric block 31 and arranged in a longitudinal direction of the dielectric block 31. The respective ends of the inner conductors 32-1 and 32-2 are exposed through the end surfaces of the dielectric block. The inner conductors 32-1 and 32-2 respectively include a wide portion 32-1a and 32-2a extending from a midpoint to a first end surface (the hidden end in FIG. 5(a)) of the block 31, and a narrow portion extending from the midpoint to the opposite end surface of the block 31 (the exposed end in FIG. 5(b)). The wide portions 32-1a and 32-2a have a lower impedance than the narrower portions.
The outer conductors 33-1 to 33-4 are formed on the top, bottom and side surfaces of the dielectric block (i.e., the surfaces which are parallel to the longitudinal direction of the dielectric block 31). The short-circuiting conductor 34 is formed on one end surface of the dielectric block 31 and short-circuits the inner conductors 32-1 and 32-2 to the outer conductors 33-1 to 33-4.
The input terminal 35-1 and the output terminal 35-2 (which are similarly constructed) are mostly formed the two side surfaces of the dielectric block 31. As shown in FIG. 5(b), the input terminal 35-1 and the output terminal 35-2 respectively include connecting conductors 36-1 and 36-2, terminal electrodes 37-1 and 37-2, and insulating portions 38-1 and 38-2 formed between the outer conductors 33-1, 33-3 and 33-4, and the terminal electrodes 37-1 and 37-2. The connecting conductors 36-1 and 36-2 respectively extend from the wider portions 32-1a and 32-1b of the inner conductors 32-1 and 32-2 to the terminal electrodes 37-1 and 37-2 through the dielectric block 31 to electrically connect the terminal electrodes 37-1 and 37-2 to the wider portions 32-1a and 32-2a of the inner conductors.
The conventional dielectric filter 40, as described above, is a distributed constant line filter composed of the inner conductors 32-1 and 32-2 and the outer conductors 33-1 to 33-4, and it constitutes a resonant circuit of a type having one end open-circuited and the other end short-circuited. That is, the exposed ends of the narrow portions of the inner conductors 32-1 and 32-2 are insulated from the outer conductors 33-1 to 33-4 while the wide portions 32-1a and 32-2a of the inner conductors 32-1 and 32-2 are short-circuited to the outer conductors 33-1 to 33-4 by the short circuiting conductor 34.
In the prior art filter 40, the dielectric block 31 is first formed, and then the inner conductors 32-1 and 32-2, the outer conductors 33-1 to 33-4, the short circuiting conductor 34, the connecting conductors 36-1 and 36-2 and the terminal electrodes 37-1 and 37-2 are formed on the dielectric block 31. For example, after the dielectric block is produced, a conductive paste is applied in necessary patterns on the surfaces of the dielectric block 31 and then the patterns are baked. Alternatively, the patterns are formed by electrodeless copper plating.
In such a conventional dielectric filter 40, the dominant coupling between the inner conductors 32-1 and 32-2 is by a magnetic field generated in the proximity of the exposed end of the wide portions 32-1a and 32-2a, which are connected by the short circuiting conductor 34. On the other hand, in the proximity of the exposed ends of the narrow portions of the inner conductors 32-1 and 32-2 (that is, at the end which is opposite the short circuiting conductor 34), the dominant coupling between the inner conductors 32-1 and 32-2 is provided by an electric field. Currents are produced in the inner conductors 32-1 and 32-2 which are induced both by the magnetic coupling and the electric coupling, and these currents are opposite in phase; therefore, these currents are completely or partly canceled by each other. In the conventional dielectric filter 40, the wide portions 32-1a and 32-2a of the inner conductors 32-1 and 32-2 are formed at the end of the filter 40 which is connected by the short circuiting conductor 34. Therefore, the magnetic coupling between the wide portions 32-1a and 32-2a dominates over the electrical coupling between the narrow portions of the inner conductors.
FIG. 7 is a block circuit diagram showing an example of a conventional high frequency circuit in which the dielectric filter 40 is used in combination with ICs 41 and 42.
As shown in FIG. 7, the dielectric filter 40 is connected to the IC 41, which forms a preceding stage, and to the 42, which forms a following stage. The dielectric filter 40 is connected through a balanced-to-unbalanced converter 43-1 to the IC 41, and through an unbalanced-to-balanced converter 43-2 to the IC 42. The IC 41 includes differential-connection transistors 44-1 and 44-2 and constant current transistors 45-1 and 45-2. Load resistances 46-1 and 46-2 are connected to the differential-connection transistors 44-1 and 44-2. The IC 41 is connected to the balanced-to-unbalanced converter 43-1 through coupling capacitors (DC blocking capacitors) 47-1 and 47-2. Similarly, the IC 42 is connected to the unbalanced-to-balanced converter 43-2 through coupling capacitors (DC blocking capacitors) 48-1 and 48-2. The IC 42 includes differential-connection transistors 49-1 and 49-2 and constant current transistors 50-1 and 50-2, and load resistances 51-1 and 51-2.
The IC 41 and the IC 42 are of a balanced-output and balanced-input type, respectively, and a large number of such ICs are used in high frequency circuits. The dielectric filter 40 is of an unbalanced-type, as shown in FIGS. 5(a), 5(b) and 6. Because the dielectric filter 40 is unbalanced, the balanced-to-unbalanced converter 43-1 is required on the input side of the dielectric filter 40, and the unbalanced-to-balanced converter 43-2 is required on the output side of the dielectric filter 40. The IC 41 includes a differential output stage including differential-connection transistors 44-1 and 44-2, transistors 45-1 and 45-2 for generating a constant current source, and load resistances 46-1 and 46-2. On the other hand, the IC 42 includes a differential input stage composed of differential-connection transistors 49-1 and 49-2, transistors 50-1 and 50-2 for generating a constant current source, and load resistances 51-1 and 51-2. In this case, when the filtered frequency band is comparatively low, it is possible to employ a wound-type converter as the balanced-to-unbalanced converter 43-1 and the unbalanced-to-balanced converter 43-2, but in general, a distributed constant line, such as a microstrip line, is used for the balanced-to-unbalanced converter 43-1 and the unbalanced-to-balanced converter 43-2.
In the conventional dielectric filter described above, a high frequency balanced signal output from the IC 41 is converted to a high frequency unbalanced signal by the balanced-to-unbalanced converter 43-1, and then the signal is input to the dielectric filter 40 to eliminate unnecessary frequency components. The high frequency unbalanced signal output from the dielectric filter 40 is then converted back to a high frequency balanced signal by the unbalanced-to-balanced converter 43-2, and it is then supplied to the IC 42.
In the conventional high frequency circuit, when a dielectric filter 40 is used in combination with ICs having a balanced output and a balanced input, it is necessary to use the balanced-to-unbalanced converter 43-1 and the unbalanced-to-balanced converter 43-2. These converters degrade the function of the high frequency circuit incorporating the dielectric filter 40 due to the insertion loss of the balanced-to-unbalanced converter 43-1 and the unbalanced-to-balanced converter 43-2. Further, a significant amount of space is needed for the converters 43-1 and 43-2, and the manufacturing cost of the high frequency circuit is significantly increased by the cost of these converters.